The present invention relates to polyethylene compositions and films thereof combining excellent optical and mechanical properties with good processability and down-gauging possibilities while assuring an excellent stiffness and shrinkage. This polyethylene composition can therefore be used for film applications, requiring this unique combination of properties, such as but not exclusively, food or non-food packaging, personal care products, agricultural or industrial products.
Low density polyethylene (LDPE) offers excellent optical properties and can be processed at low temperatures and pressures while maintaining a good melt strength. LDPE has however limited possibilities for down-gauging, due to a low draw ratio, and a low stiffness.
Linear-low-density polyethylene (LLDPE) has greatly improved down-gauging possibilities and excellent tear and impact properties; its stiffness however remains low and its processability is well below that of LDPE. Also, conventional LLDPE""s optical properties do not match those of LDPE. Optical properties of LLDPE have been improved by using metallocene-catalysed LLDPE (mLLDPE) resins; stiffness is however not improved in these products and the processability of these grades is generally worse than that of conventional LLDPE.
Wherever high rigidity is needed, LDPE and LLDPE compositions will require overly thick structures. Especially for LLDPE, where excellent impact and tear properties render its down-gauging capability useful, the lack of rigidity is a main drawback because high rigidity is a requirement for product handling.
Conventional medium density polyethylene (MDPE) offers an improved rigidity, excellent processability and down-gauging possibilities. MDPE however lacks the good optical properties of LDPE or LLDPE.
Blends of conventional MDPE with LDPE and/or LLDPE can be used for improving rigidity and/or down-gauging and/or processability of LDPE and/or LLDPE but do not provide their good optical properties. WO 95/27005 discloses mixtures of LDPE with LLDPE or mLLDPE. The stiffness of their resins is insufficient.
Therefore, there exists a need for a polyethylene composition that will match the transparency and gloss offered by LDPE and the stiffness of MDPE while maintaining the same impact, tear, down-gauging and processing properties as MDPE.
It is therefore an object of the present invention to provide polyethylene compositions for films, prepared with one or more layers, that achieve a better balance between the good optical qualities of LDPE and the rigidity, down-gauging, processability, impact and tear properties of MDPE.
It is another object of the present invention to provide polyethylene compositions that can be used to prepare films with improved shrink properties.
In the present invention, a film is defined as an extremely thin continuous sheet: the upper limit for thickness is of about 250 microns (Hawley""s Condensed Chemical Dictionary, Twelfth Edition, Rev. by R. J. Lewis, Van Nostrand Reinhold Co., New York)